Do I need to be concerned with race conditions with asynchronous Javascript?

Question

Suppose I load some Flash movie that I know at some point in the future will call window.flashReady and will set window.flashReadyTriggered = true.

Answer 1

Yes, of course there are race conditions in Javascript. It is based on the event loop model and hence exhibits race conditions for async computations. The following program will either log 10 or 16 depending on whether incHead or sqrHead is completed first:

const rand = () => Math.round(Math.random() * 100);

const incHead = xs => new Promise((res, rej) =>
  setTimeout(ys => {
    ys[0] = ys[0] + 1;
    res(ys);
  }, rand(), xs));

const sqrHead = xs => new Promise((res, rej) =>
  setTimeout(ys => {
    ys[0] = ys[0] * ys[0];
    res(ys);
  }, rand(), xs))

const state = [3];

const foo = incHead(state);

const bar = sqrHead(state);

Promise.all([foo, bar])
  .then(_ => console.log(state));